This invention relates generally to systems for monitoring capacitors batteries, and more particularly, to a system for monitoring a capacitor battery in an LC filter circuit of an AC power network, the capacitor battery being of the type formed of a plurality of parallel legs each having a plurality of sectional capacitors connected in series with fuses, adjacent ones of the sectional capacitors being connected to one another by cross lines.
The generation of multi-phase rotary voltages, such as are produced by converters, produce a line frequency fundamental component on which are superimposed characteristic harmonics. Filtering is generally achieved by using filters which are tuned to the frequencies of the characteristic harmonics. At the fundamental frequency, such filter circuits appear as capacitive impedances.
The capacitances of filter circuits are generally formed of a capacitor battery having a plurality of parallel legs, each such leg having a plurality of sectional capacitors which are each connected in series with a respective fuse. Sectional capacitors in adjacent legs are interconnected by continuous cross lines. Thus, a multiplicity of sectional capacitors may be contained within a capacitor battery. In fault situations where a sectional capacitor short circuits, adjacent ones of the interconnected sectional capacitors discharge via the cross lines to the fuse which is connected in series with the defective sectional capacitor, causing the fuse to open circuit. Such a failure of a sectional capacitor causes the overall filter circuit to become detuned. However, such detuning results in only slight change in the filter current at the fundamental frequency component, thereby rendering detection of defect in a single sectional capacitor to be difficult to perform with known current monitoring and measuring systems. This problem is further compounded by the fact that variations in the filter circuit resulting from changes in the ambient temperature may be greater than the current changes which occur as a result of the failure of one or more sectional capacitors.
Although the harmonic content of the filter current changes substantially more than the fundamental component when a sectional capacitor fails, such variations in the harmonic content of the filter current are not suitable for evaluation to determine the failure of sectional capacitors because the harmonic content varies greatly with the control angle of the converter. It is therefore impossible to determine whether a change in the harmonic content of the filter current is caused by a control angle change in the converter or by a defective sectional capacitor in the capacitor battery of the filter circuit.
It is, therefore, an object of this invention to provide a monitoring system for an LC filter circuit in an AC power network which is simple to construct and reliably indicates the failure of one or more sectional capacitors.